Glass optical fibers, cables, and sensors have proven advantageous over traditional copper-based data transmission media; and have found many uses in distributed sensing as both sensor and signal transmission link. These uses often are in high temperature harsh environments, such as oil wells or under-hood automotive applications.
The thermal stability of the optical fiber is a critical functional property for the success of the use of the optical fiber in the high temperature harsh environment. The coatings on an optical fiber have thermal stability issues relating to their composition and the heat stability of the components of the composition.
The state of the art coating for optical fiber in high temperature harsh environments includes hermetic carbon coatings, other thermally cured coatings comprising reactive silicone resins or polyamides, silicone resin based UV curable coatings as described in U.S. Pat. No. 5,891,930 and certain other UV curable coatings as described and claimed in U.S. Pat. Nos. 4,741,958, 5,977,202, 6,362,249, 6,438,306, 6,714,712, 7,174,079, and 7,276,543.
In Japanese Patent Application, JP63006507, coating materials contain a photopolymerization initiator and a reaction product of epoxy resin, dicarboxylic acid of long-chained aliphatic compound, and acrylic acid or methacrylic acid. The materials have flexibility, short hardening time, and good adhesion to optical fibers. Thus, Epon 828 (Bisphenol A diglycidyl ether), SB-20 (C20 fatty acid dimer), acrylic acid, triethylamine, and hydroquinone were reacted to give a reaction product (number average mol. weight 2000) which was mixed with 2-hydroxy-3-phenoxypropyl arylate and benzoin iso-Butyl ether to give a fiber coating material having low viscosity. Optical fibers coated with the material had a uniform surface and high strength. No mention is made in this published Japanese Patent application of the High Temperature Resistance of these cured coatings.
In Japanese Patent Application, JP59074507, Optical fibers are prepared with 2 coating layers wherein the 1st layer, closest to the core glass fiber surface, is composed of a resin composition (˜5μ thick) containing a nonsilicone type parting agent. This coating on the optical fiber can be removed easily and hence the fiber can be connected to other optical fibers by simple operations. Thus, a glass rod (for optical fiber) 30 mm in diameter was drawn at 2100° C. to form a fiber 125μ in diameter, coated with a UV-hardenable epoxy acrylate (1.5μ thick) containing stearic acid 1.0 weight %, irradiated by UV radiation to form a hardened layer, coated with dimethylpolysiloxane (80μ thick), baked by IR radiation, and coated with nylon 12 to give the optical fiber. The layer was completely removed when the nylon 12 was removed with a tool. No mention is made in this published Japanese Patent application of the High Temperature Resistance of these cured coatings.
These types of coatings have advantages and disadvantages. However, to date radiation curable coatings have been found to enjoy very limited use in high temperature environments due to the limited function at temperatures of 100° C. or higher for an extended time period.
There are commercially available optical fibers that are advertised as “High Temperature Resistant Fibers” such as Draka Elite High Temperature BendBright XS, which is described on their website as having a “High temperature resistant Acrylate coating”. The website is silent about further details concerning the coating used on these High temperature resistant Optical Fibers so there is no information available as to exactly what the compositions are, nor what the functional properties of the compositions are, nor what the % weight loss is of these coatings after 100 hours at 180° C.
It would be desirable to have access to a radiation curable coating for an optical fiber that upon curing achieved a resistance to thermal degradation such that the optical fiber was still viable as a communications medium at elevated temperatures.